if Joseph had only lived as a child and died at 5 years old, would that mean that a generation can be only 5 years long? good questions. No, according DS's reasoning here, even when that child was born the generation was already ~90 years and counting . By the time Joseph died the generation was already 220 years, would reach ~ 300 years if the +- 10 minute birth/death did not happen.
Splane's example implies 3 groups in the overlap and makes for a 300 year generation of Joseph. and: Yes you can have an overlapping generation with a 5 year old father, because virgin birth would have been possible, even if the other Joseph was only 5 in Bethlehem.